In recent years it has been a goal of lift truck manufacturers to provide a lift mast, of the type used on a fork lift truck, with improved visibility and free lift. U.S. Pat. No. 4,219,302 dated Aug. 26, 1980 to Edward V. Leskovec and U.S. Pat. No. 4,191,276 dated Mar. 4, 1980 to Stanley E. Farmer disclose a lift mast having improved visibility and free lift. Free lift is well known in the art as elevational movement of a lift carriage prior to extension of a movable upright assembly past the upper end of the fixed or stationary upright assembly. Although these two designs both accomplish improved visibility and free lift they both require additional parts which tend to be complex in nature and add to the cost of the lift mast.
It is preferred that high visibility be achieved without any additional structure or parts while maintaining free lift as a feature thereof. In order to achieve this it has been determined that the movable lift mast uprights and particularly the upper end thereof be spaced elevationally beneath the upper end of the fixed uprights so that the movable uprights and particularly the carriage can move to a preselected elevated face lift position, from a fully lowered position, without the upper end of the movable upright passing the upper end of the fixed upright.
Typically, lift masts of this type require upper and lower guide bearing assemblies, such as rollers, to be provided between the fixed and movable uprights so as to guide the uprights smoothly so as to prevent sticking, bending, excessive wear and the like. Also, it has been accepted that the distance between the upper and lower guide bearings should be spaced as far apart as possible to offer the maximum amount of movable upright extension relative to the fixed upright for a given length as possible. However, placement of the movable upright beneath and spaced from the upper end of the fixed upright requires the upper guide roller to be moved downward to a location to support the upper end of the movable upright when in its lowered position or provide an additional guide roller adjacent the upper roller.
This added intermediate guide roller creates an additional series of problems. The total overall maximum height of extension of the movable upright assembly is reduced, separation of the mast sections for disassembly and maintenance are further complicated in that the cutout of the flange at the upper end portion of the fixed upright must be increased in length resulting in weakening of the strength of the fixed upright sections, and alignment of the upper and intermediate guide rollers to provide proper contact between the movable upright and the rollers to reduce premature wear, failure and alleviate the potential of binding of the movable uprights.
The present invention is directed to overcoming one or more of the problems as set forth above.